Electron multiplier



Sept. 2, 1941. D. GABOR 2,254,422

ELECTRON MULTIPLIER Filed May 26, 193'? 3 Sheets-Sheet 2 j INVFNTOR06777715 6427502" I BY W. Dam mqs'pmz,

ATTORNEYS Patented Sept. 2, 1941 ELECTRON MULTIPIJER Dennis Gabor,Rugby, England Application May 26, 1937, Serial No. 144,787

- In Great Britain May 27, 1936 13 Claims.

This invention relates to electric discharge devices, known under thename of electron multipliers, in which a primary electron stream isamplified by secondary electron emission. The primary electron stream,which may be produced by photoelectric or by thermionic emission,impinges on an electrode to be called a secondary cathode. The secondaryelectron stream emitted by the secondary cathode impinges in turn on 'asecond secondary cathode, maintained at a higher potential than thefirst secondary cathode, and this process is repeated until thesecondary electrons emitted by the final secondary cathode impinge on acollecting electrode or anode. If each secondary cathode emits more thanone electron for every impingingelectron, and the number of steps orstages is chosen sufficiently high, the current collected by the anodeor output electrode may be a very high multiple of the primary current.

.-One object of the present invention is to provide an improvedarrangement of electrodes suitable for a high degree of amplification ofphotoelectric or thermionic currents by means of electronmultiplication.An important feature of this electrode arrangement is that the secondarycathodes are surfaces of rotation, arrayed along an axis of symmetry. Afurther feature is that one or more axially symmetrical electrodes, tobe called auxiliary anodes, are provided inside the secondary cathodes,in order to produce a substantially radial electric field in addition tothe substantially axial electric fields produced by the successivelyrising potentials of the secondary cathodes. A further feature is, thatmeans are provided; to produce a substantially axial magnetic field inorder to prevent the electrons from hitting the said auxiliary anode oranodes.

Another object of the invention is to provide improved means for theamplification of thermionic currents by utilizing electronmultiplication. 7 A novel method for controlling the output current isvprovided by means of modifying the gain of said electronmultiplication.

The novel features which I desire to protect herein will be pointed outin the appended claims. The invention may be best understood byreference to the following description and the accompanying drawings. Inthe drawings Figs. 1, 2 and 3 are diagrammatic representations,explaining the principle of the invention. Figs. 4 and 5 showalternative modifications of va device according to the invention. Figs.6 and 7 are illustrations of certain improved details in the electrodearrangement. Fig. 8 is a suitable arrangement of magnetic coils whichmay be used in connection with the device according to the invention.Figs. 9 to 19 are diagrammatic representations to explain dimensioningand additional aspects of the device according to the invention. Figs.20 to 23 relate to a novel method for modifying the gain of the device.Figs. 24 and 25 show methods of controlling the intensity of athermionically produced. primary beam. Fig. 26 illustrates the circuitsof an electron multiplier according to the invention, whereby it may actas a complete superheterodyne set for radio or television reception.

The principle of the new electron multiplier is shown in Fig. 1, whichis a diagrammatic longitudinal section of a schematically simplifieddevice. This is understood to be enclosed in an evacuated envelope 3. Iand 2 are two cylindrical bodies which have potentials linearlyincreasing with the distance 2: measured from the top edge of I. Inevery cross section the potential of 2 shall be greater than of I, andthis potential difierence may be called V. V can be independent of z orvary to a certain extent which shall be specified later. In the spacebetween I and 2 the'electric field is therefore composedof a radialfield with the intensity er, the intensity of this field varyinginversely with the distance r from the axis, and of a longitudinal fieldof the intensity Ez. To this composite electric field is added accordingto the invention a longitudinal magnetic field of the intensity H. I

The electrodes in these and subsequent figures are supported within theevacuated glass envelope 3 by suitable means which are not shown in thedrawings in the interest of clearness of illustration.

The electric field as described can be realized by making both I and 2of materials with high resistance, through which currents may be passedin an'upwa-rd direction. Constructions which are more convenient forpractical purposes shall be described later. The magnetic field may beproduced for example by means of a long coil 4 placed coaxially aroundthe vessel, or by a tubular permanent magnet, made of a suitablematerial such as cobalt steel.

If by photoelectric'action or by other effects slow electrons arereleased from the surface of I, they will start moving towards thecentral cylinder 2. They will be, however, deflected by the magneticfield and forced to return to I.

This is illustrated in Fig. 2, which shows the projection of theelectron paths on a plane page pendicular to the axis. In this figure, Iis the outer electrode, to be called the cathode, and 2 the inner one,which shall be called auxiliary anode. Fig. 3 shows the paths of theelectrons in circular projection on a plane passing through the axis. Inthese two figures it has been assumed for simplicity that the fieldcomponents er, Ga, and H are independent of 2. It will be seen that theelectrons return to the cathode afterhaving travelled a verticaldistance Z, which shall be called the step length. branches of the orbitin Fig. 2 are symmetrical,

The two the electron takes equal times for approaching the innerelectrode to a minimum distance and for returning to the cathodesurface. As however each electron travels vertically with a uniformlyaccelerated motion, it will travel one quar ter of the step length intherfirst halfof the time and three-quarters in the secondhalfa .For Ithis reason the two branches of the orbit as they appear in thelongitudinal projectionin Fig. 3' will be strongly, asymmetrical.Assuming that the electron has started with zero velocity, itwill-return tangentially to the cathode. If the voltage drop along thestep length'is sufficiently high, it will release secondary electrons.These will start again with very low velocities and describe paths ofthe same shape, until the last electron leaves the space between thecylinders.

If the number of secondary electrons released by one electron is greaterthan unity, the device separated from one another byfslits Y. The

cathode surfaces preferably comprise materials suitable for secondaryelectron emission such as The figure shows a longitudinal caesium.Coaxially arranged with the secondary I cathodes, there are provided anumber of cylinders,;numbered ll,.l2, l3, Hand is, telescopi callyassembled, The last of these, [5, is connected with the collecting oroutput anode, the flare I6, which collects the electrons releasedatthelast cathode l0. 7 j V J V 1 The secondary cathodes and auxiliaryanodes all have connections with the outside and may be maintained atdifierent potentials with respect to one another. In the arrangementillustrated each anode section is connected with the following cathodesection, I] with 1, 12 with 8,

l3 with 9 and I4 with In. Co'nsequentlyif. equal potential difierences Eare impressed between successive cathodes, there will be aconstant-potential difierence between each cathode and the correspondingauxiliary anode. ;It should be understood however that the stages neednot. be equal in length or voltage, nor need the radial potentialdifierences be equal to the longitudinal n t ntiel li eren,. e -v '-.demete i thwe Q e' c l nders-m ei ryrm a i st ge and flier. ma b .ea oncal nsiead yli r drical. For making; the illustrations simpler {I havhowev as umedqua and i m ic st s. men th eceom enyin d aw I Eisv,sh9ws..-aa. n t ye ccnst e i Jl'h aw lierranodeis her f a z r er.wch-meme m de o a were: i high spe ic mine e r i may be.e-ja se eie;insula in t a we r w hs ch am ieria ai iisisfii fid w h n' it; 1815'iiansil qw iie divi ed? ance into sections. These are connected by meansof thin wires 2|, 22, 23 and 24 with the cathode sections 26, 21, 28 and29. The last ring is connected with the anode flare 25.

In the foregoing constructions the electric field at the ends of theelectrode array is necessarily different from that at the centralportions. This can be avoided as shown in Fig. 6 by giving the firstcathode 30 and the anode flare 3| shapes identical with those ofthepotential lines which would exist if the electrode column wereinfinitely long. Their shape can also be simplified as shown in Fig. 7,where the end plates 32 and 33 are composed of frusto-conical,cylindrical and plane sections.

In some cases it is advantageous to have a coil which produces themagnetic field constructed in such a way that it produces a homogeneousfield over the whole volume of the electron multiplier. This has theadvantage that in a homogeneous field the relative position of the coiland the device need not be adjusted very accurately, Fig. 8 shows aconvenient arrangement. It consists of three single coils 34, 35 and 36with dimensions which are small as compared with their diameter D, andspaced at the distance D in the axial direction. If. the number ofampere turns of 34 and 36 are equal, and the number of ampere turns ofthe central coil 35 is 11.6 percent of that of 34 or 36,'the fieldstrength along the axis will vary by less than i 1 percent.

The most advantageous dimensions of the device can be obtained byconsidering the relation which links up its various factors. Thisequation can be written in the form:

Here Z is the step length, i. e. the distance in cms. which an electronstarting with zero velocity travels in the direction of the axis beforereturning to the outer cylinder. This must be approximately equal to thestage length X+Y, if the device is to work as a multiplier. H is themagnetic field in gausses. E is the potential difference between twosuccessive cathodes, in volts. R is the radius of the cathodes, in cm.

7 V is the potential difference between the cathode and the oppositeauxiliary anode. R1 is the radius ofthe'auxiliary anode. The nature ofthe function f is represented in Fig. 9. It will be seen that thefunction f has a maximum. Thismeans, that the step length Z has amaximum for acertain value of V-in an otherwise determined arrangement.Choosing the operating point at or near this maximum affords theadvantage, that small variations of V or of R1 are'of no importance.This is'shown more particularly in the following numerical example:

According to Fig. 9 the maximum occurs approximately at We can choosefreely 4 quantities, e. g. R=1.5 cms.,' R1'='0.5 cm., .Z 1 cm. and E='volts. This gives H 22.5"'gausses and 1V=53.5" volts. Because of themaximum this voltage need not bebbservedvery' exactly. Z'will beshortened only by? 1% or 0.1mm. if 'V is 4501 62 volts, or Rile'q'ualj1:00.42 or 9.6 cm. It will. be seen that this mode I of operation is'particularly advan ta eous for devices ofth kind as shown. in Fig. 4,iasj no correctiohs'j heed to, be introduced for the varying diameter orthe auxiliary anodes.

Undr som conditions itLmight be however advantageom ochoose the workingconditions so as to remain at the left of the maximum. Near to theorigin the function f approaches a straight line, and the equationcan bewritten approxi- This means that for weak magnetic or very strongelectric fields the step length Z depends only on the ratio 'of theradial and longitudinal potential steps. This can be understood fromFig. 10, in which the electron path a corresponds to a weak magneticfield. The length of this path is very nearly equal to the diameter onwhich the electron would move in the case of a zero magnetic field. Thismode of operation has the great advantage, that the step length isdetermined entirely by the electrical connections. The magnetic fieldhas only the function of hindering the electrons from hitting theauxiliary anode. This mode of operation is particularly advantageous inthe case of thin auxiliary anodes like in Fig. 5.

In the same drawing is also shown a path b which corresponds to themaximal step length. The electron approaches the axis to a minimaldistance equal to about 60% of the outer radius. The auxiliary anode canhave therefore a rather large diameter, without incurring the risk ofthe electrons hitting it.

As compared with other electron multipliers, axially symmetrical devicesaccording to the invention have the advantage that lateral scattering ofthe electron paths has no consequence and no cautions need be taken toavoid it. Scattering in the axial direction might however causeinconveniences as it might prevent a part of the electrons from reachingthe last cathode. Such an effect is-produced by the inhomogeneity of thelongitudinal electric field. Instead of being constant as assumed above,the field is zero along the surfaces of the cathodes and very strong inthe slits between them. This produces a focussing effect as shown inFig. 11. An electron which starts from the lower edge of a cathode 31comes at the beginning of its path P1 under the influence of a strongfield. We assume that it reaches the corresonding edge of the nextcathode 38. An electron however which starts from the top edge of 38will move at the beginning of its path P2 in a weak field and might landat the same spot as P1. Electrons returning to the same cathode are ofcourse wasted. This can be prevented according to the invention by aninhomogeneous magnetic field, which can be produced, as shown in Fig.12, by putting wedge shaped rings 34, 40 of high magnetic permeabilitybehind the oathodes il, 42. The cathodes themselves can be also wedgeshaped and made of a material with considerable permeability like nickelor nickel-iron alloys. The path Pi which starts from the edge of 4|crosses therefore a stronger magnetic field than the path P'z. Bychoosing a suitable shape and size for the rings 39, 40 it is thereforepos- -to become suddenly homogeneous.

of small diameter in order to avoid obstruction of light.

Tubes of large surfaces areconvenient for certain applications ofphoto-tubes, especially ifthe light can not well be concentrated byoptical means. Figs. 14 and 15 show a construction of the electronmultiplier according to the invention, to be used as a self-amplifyingphoto-tube. The first cathode 46 is longer than the following secondarycathode 41. In order to prevent distortion of the electric field, thephoto-cathode 36 is completed to a cylinder by a grid or gauze s8, whichcovers the window through which the light falls in, but does not cut offtoo much of the light., The first auxiliary anode 49 is preferably athin wire, which may be fastened by some insulating member to the topplate 50.

In this case however special measures are necessary in order to preventthe electrons returning to the photo-cathode. This can be done accordingto the invention by an inhomogeneous magnetic field, the effect of whichis shownin Figs. 16 and 1'7. Fig. 1'7 shows in a plane perpendicular tothe axis the effect of a magnetic field which increases or decreasesalong the axis z. An increasing field will produce a stronger curvaturein the returning branch of the curve, so that the electron will turnback before reaching the cathode and will go on moving one kind ofepicycloidal path. A decreasing field on the contrary will not turn theelectron back by a full so that the electron aproaching the cathodeunder a grazing angle will not be able to reach it and will go on movingonv a kind of hypocycloidal path. The practical result is however inboth cases the same. Both increasing and decreasing fields will preventthe electrons from returning to the cathode. Fig. 16 which shows thepath in circular projection on a meridian plane corresponds therefore toboth cases. Such inhomogeneous fields can be produced easily either byplacing or dimensioning the magnetic coils conveniently or by backingthe first cathode with wedge shaped rings of ferromagnetic material.

This measure would have, however, the consequence that the electronswhich have departed to a considerable distance from the first cathodewould not come back to the second cathode, i. e. the first secondarycathode, even if the field were They can be brought back, however,according to the invention, by a decreasing radial electric field, i. e.by making V to decrease with increasing 2. The effect of a decreasing Vis that the electron will not have lost all its radial velocity when itreaches again the radius R, and that therefore the path has a tendencyto move outwards. This is shown in Figs. 19 and 18-,where also thevirtual prolongation of the path is represented by dotted lines.

The decreasing radial field can be produced by choosing the lengths ofthe anode sections and their connections with the cathodesections'conveniently. The simplest way however is to employ only asingle auxiliary anode, such as a solid rod, tube or wire, extendingalong the whole length of the device. This auxiliary anode may.

be connected with the last or collecting anode,

or with a suitable, potential higher than the 7 advantage that theelectrons do not strike the cathodes any more under grazing angles,;"and

their paths are consequently. less strongly modispace charges at-highercurrent densities. M r .f If the device is to be used as a simplephototube, it is important to make its response independent from thepointof incidence of the light beam, by the means as. explainedinconnection with Fig. 14. The newdevice is, however, particularlysuitable also for other applications, in

which the output current is deliberately made,

will reach the last circuit only if 20 is larger than a certain minimum,otherwise'they will return to the last cathode Stinsteadof reaching theanode 52. We call the first current, which can be photo-electric, I0,and the current between the last cathode and the anode, I. Fig. 21showsthe amplification factor or gain I/Io as a function of the startingpoint of the electrons, 'It is zero below a certain distance and aboveit jumps suddenly to a finite value, This will be the case, of course,only if the electrons start from one point; in the case ofphoto-electrons, for example, if the light spot from which theyoriginate is infinitely small. With a spot of finite width the slope becomes finite, as'shown in Fig. 22, the length of the slope S being equalto the spot width. In this range the amplification factor is a linearfunction of the spot position, if the light density is homogeneous. Thisphenomenon can" therefore be used, for example, in conjunction withmirror galvanometers, with instruments for precision measurements ofsmall movements, for sound registration and reproduction, etc- Theelectron emission from the first cathode ring can also be produced by abeam of electrons instead of by a light beam. Thenew electron multiplieris particularlysuitable as a thermionic amplifier because of itsaxialsymmetry. In Fig. 23, 53 is an indirectly heatedthermionic cathode witha heating filament 54. 55 and 56 are two cylindrical tubes, formingbetween themselves an annular slit 51. The potential of 55 and 56 ishigher than of the first cathode 58 and the potential of 53'is lowerthan that of 58 by about an.

equal amount. The beams are therefore ejected from the slit with arather high velocity and somewhat retarded in the outer space. It can beshown that the magnetic field willnot prevent these primary electronsfrom reaching the cathode ,58, whereas it will be strong enough forkeeping the secondary electrons away from the inner system. H a I Twoplane rings 59 and: 60 fittedoutsidethe slit are serving asdeflectingplates. I By impress.- ing a variable potential between them, the beamcan be deflected in theagial directionbBy adjusting the dimensionsconveniently, deflections of the order of 0.17-1.0 mm. /voltcanbeobtained. This can result in variations of the final current of theorder of several milliamperesper voltgwhich is of the order ofthejfslopes 'ofthe .DIdi-i nary thermionic valves. Theamplificationffactor fmu is extremelyhigh, as thegvoltage of .the outputanode has hardlyany effect on thecurrent.

This kind of control shall be called deflection .contro ,fordifferentiating it from intensity control, as shown in Fig. 24. In thissecond kind of control the intensityof the beam is varied, withoutchanging its position. In Fig. 24, 6| is a thermionic cathode. This isfitted with two grids, 62 and 63, so as to form a small thermionic valvewith a perforated anode. 62 is an accelerating grid, 63 is the controlgrid. This is fitted with side. rings 64 in order to prevent theelectrons from flying to the next auxiliary anode section 65.

The electrons fly through the meshes of the "anode grid 62'to the firstsecondary cathode 66,

from there to 61 and so on. It is a particular advantage of thisarrangement that the field between, the cathode 66 and the anode grid 62is completely separated electrically from the small thermionictube whichis serving as electron tained, withoutany perceptible anode reaction,

as the currents flowing in the first stage are extremely small. It isadvisable to use self-focussing arrangements; like the one shown in Fig,12, in order to utilize the whole surface of the secondary cathodes,especially in the later stages, where the currents are considerable.

Intensity control and deflection control can be also combined as shownin Fig. 25. Here 68 is the thermionic cathode with the filament 69; 16the controlling electrode, which is a small cylinder with a slit; H theaccelerating electrode; 12 and 13 are the two deflecting plates. As theposition of the beam can be controlled by either of the .defiectingplates, this device contains three possibilities for applyingcontrolling voltages. This arrangement has also the advantage that thecontrolling and the accelerating electrode can be adjusted in such a wayas to concentrate the beam at the secondary cathode 14 ina narrowangular zone.

A thirdmanner of control is control by the longitudinal magnetic field.This can be a very sensitive control if the initial beam is narrow andno self-focussing measures are applied. The sensitivity depends also onthe position of the working point on the curve in Fig. 9. Near itsmaximum Z and H are simply reciprocal, i. e. 1% increase of H causes 1%decrease in step length. If there are e. g. 10 stages, this means ashifting of the position of the last spot of incidence by 10% of thestage length. We see therefore that it .is possible to produce veryconsiderable current variationswith magnetic fields of less than 1gauss. In order to make highfrequency. control. possible, thecathode'rings must be split.

,A further possibility of control according to the invention iselectrostatic control after the last stage. This will. be explained inconnection withan example, in which I intend to show, that the newelectron multiplier can perform, according to the invention, all'thefunctions which are performed by valves in superheterodyne receiversets,

it p ifi requency conversi r c ification (demodulation), and automaticvolume control.

Fig. 26 shows the diagram of an electron multiplier with circuitconnections and additional circuit elements, in which the multiplierperforms all the functions of the several tubes in a superheterodynereceiver, for sound. broadcasting or television reception, without anyother tubes.

The signal is applied to the control grid '15 by means of the couplingresistance T6; This point is marked by H. F. which means the highfrequency of the carrier wave. The aerial circuit is not shown in thefigure. The construction of the thermionic system for the emission ofthe primary electrons is the same as in Fig. 25 the previous figure. Itconsists of a thermionic cathode 11, a control electrode 15, anaccelerating electrode 18 and deflecting plates 19 and 86. Onedeflecting plate, 79, is used for heterodyning (frequency conversion)the other, 88, for automatic volume control. The plate 19 is energizedby'a local oscillator 96, labeled L. 0., by means of regenerativeoscillations with a frequency To. These are produced by picking up theoscillations at one of the intermediary secondary cathodes, 8! in thefigure, and feeding them back to the plate 19 across the tuned reactorcircuit marked L. O.

The action of the heterodyning plate can be understood from Fig. 22. Thevoltage of the deflecting plate controls the factor by which theelectron current is amplified and produces with the original frequencyin the two beat frequencies fh+fo and fir-fotermediate frequency fi. Itmay be conveniently about 10% of the original frequency fn. This in--termediate frequency is filtered out and amplified by selectiveregeneration. It is picked up at one of the intermediate secondarycathodes, 82, and fed back to the control grid through the reactorcircuit 91, marked I. R, which is tuned to the intermediate frequencyf1. This frequency is now again amplified and also heterodyned, so thatthe signal received at the final stage is a modulated wave of threecarrier frequencies fh, fhfo and fh2fo. This has however no detrimentaleffect on the selectivity of the set, as all three oscillations arederived from and proportional to the oscillation with the intermediatefrequency which has been filtered through the circuit I. F. Theregeneration of the intermediate frequency ensures moreover a highdegree of modulation throughout the device, and therefore makes itpossible to utilize the current capacity of the device to its limit. Afurther advantage is, that the conversion conductance can be kept ratherlow and harmonic distortion can be avoided. g

The final stage of the device consists of the last secondary cathode,83, the anode 8 and three grids 85, 86 and 87. The first grid is placedabout one step length below the middle of the last cathode. Theelectrons have here velocities nearly in axial direction. This has theadvantage that as they move through the grids 85-31 they will beproceeding in the direction of the magnetic field, which therefore willnot interfere with their movement.

This final system can be used in various ways. It could be used,,e. g.,for frequency conversion, if an additional valve were to be used for theloudspeaker or other output device. In the present example I intend,however, to show that the new electron multiplier is a self -containeddevice The latter may be called in and explain-the use-of the finalstage as a rectifier, (current detector or demodulator).

The grid 85, which has the same potential as the guard ring 88, i. e.the potential which the next secondary cathode would have if the systemwere continued, is acting as an accelerating (space charge) grid. Itspotential is derived from a potentiometer 89.. The control grid 86 has apotential only little higher than the last cathode. For simplicitys sakethe grid bias is shown in the figure as being derived from the samepotentiometer 89. Between anode and control grid is placed the anodescreen grid 81.

If the control grid is conveniently biased, its action will be to cutoff the anode current beyond a certain current value and make it flowback to the space charge grid 85. The rectified current can be derivedeither from the anode or from 85. In the figure the loudspeaker 9%) isenergized from the anode across a transformer 9|. In the case oftelevision the modulating electrode of the cathode ray tube is to beconnected with 34 or through a suitable bias.

Automatic volume control (A. V. C.) is effected in the following way.The reaction with the filtered intermediate frequency is applied notonly to the control grid, but also, through the condenser 98, to thesecond deflecting plate 80 opposite to the plate 19 which is used forheterodyning. The plate 80 is, however, placed so near the electronbeam, that at positive potentials it would attract a sufficient numberof electrons for neutralizing its charge. It will assume therefore anegative charge so as just to repel nearly all electrons, i. e. it willassume a charge proportional to the high frequency amplitude and producea corresponding deflection of the beam in the upward direction. Thegreater, therefore, the intensity of the high frequency signal, thenearer the top of secondary cathode will be the starting point of theelectrons, and this will result, according to Fig. 22, in a smallerampli fication factor.

Fig. 26 shows also the system of potential dividers, formed byresistances 92, 93, 94 by which the suitable potentials are applied tothe secondary cathodes.

While I have shown particular embodiments of my invention, it will beunderstood that many modifications and applications may be made by thoseskilled in the art without departing from the invention as set forth inthis specification and in the appended claims.

What I claim as new and desire to secure by Letters Patent of the UnitedStates is:

1. An electron multiplier comprising a plurality of rotationallysymmetrical cathodes having inner surfaces capable of emitting secondaryelectrons, said cathodes being coaxial and axially spaced from oneanother forming a column, an anode of rotational symmetry inside thecolumn of said cathodes and coaxial with them, an output anode at theend of said column, and means independent of said plurality of cathodesfor producing a magnetic field of substantially axial direction.

2. An electron multiplier comprising a plurality of rotationallysymmetrical cathodes hav ing inner surfaces capable of emittingsecondary electrons, said cathodes being coaxial and axially spaced fromone another, a plurality of axially separated anodes of rotationalsymmetry inside the column of said secondary cathodes and coaxial withthem. the axial separation of said anodes being substantially the sameas the axial separation of the secondary-emissive cathodes, an outputanode at the end of said column, and means for producing a magneticfield of substantially axial direction.

3. An electron multiplier for the amplification of photoelectriccurrents, comprising a plurality of cathodes capable of emittingsecondary electrons, said cathodes having rotational symmetry and beingcoaxial and axially spaced forming a column, a photoelectric cathode ofrotational shape at one end of said column and caxial therewith, atleast a portion of said photoelectric cathode being foraminous to admitlight to the inner surface thereof, an anode of rotational symmetryinside said column and coaxial with it, an output anode at the end ofsaid column, and means independent of said plurality of cathodes forproducing a magnetic field of substantially axial direction.

. 4. An electron multiplier for the amplification of'photoelectriccurrents, comprising a plurality of .cathodes capable of emittingsecondary electrons, said cathodes having rotational symmetry and beingcoaxial and axially spaced forming a column, a photoelectric cathode ofrotational symmetry at one end of said column and coaxial therewith, atleast a portion of said photoelectric cathode being foraminous to admitlight to the inner surface thereof, a plurality of axially separatedauxiliary anodes of rotational symmetry inside said colurrm, an outputanode at the end of said column, and means independent of said pluralityof cathodes for producing a magnetic field of substantially axialdirection.

5. An electron multiplier for the amplification of thermionic currents,comprising a plurality of electrodes capable of emitting secondaryelectrons, said electrodes having rotational symmetry and being coaxialand axially spaced forming a column, a therminoic cathode of rotationalshape in the axis of the column at one end of it for supplying electronsto the secondary-electron emissive electrode at. said one end, saidthermionic cathode being surrounded by an electrostatic controllingelectrode of rotational symmetry, an anode of rotational symmetry insidesaid column and coaxial with it, means for producing a magnetic field ofsubstantially axial direction and a collecting output anode at the otherend of said column.

6. An electron multiplier for'the amplification of thermionic currents,comprising a plurality of 'electrodes capable of emitting secondaryelectrons, said electrodes having rotational symmetry and being coaxialand axially spaced from one another forming a column, a thermioniccathode of rotational shape in the axisof the device at one end of itfor supplying electrons to the secondary-electron emissive electrode atsaid one end, said thermionic cathode being surrounded by electrostaticcontrolling electrodes of rotational symmetry, a plurality of axiallyseparated anodes of rotational symmetry inside said column and coaxialwith it, means for producing a magnetic field of substantial axialdirection and a collecting output anode at the other end of said column.

7. An electron multiplier including a plurality of coaxial axiallyspacedsecondary-electron emissive electrodes of rotational symmetry,means for producing an electrostatic field of radial direction, meansfor producing a magnetic field of substantially axial direction, andmeans for producing a desired degree of inhomogeneity in said magneticfield, comprising suitably shaped ing a column, means for producing anelectrostatic field in radial direction, means for producing a magneticfield of substantially axial direction, a thermionic cathode positionedand adapted to emit electrons to the secondary-electron emissiveelectrode at one end of said column, and means for deflecting theelectrons'emitted by the thermionic cathode to diiferent regions of thesecondary-electron emissive electrode at said one end of the column.

9. An electron multiplier including a plurality of coaxial axiallyspaced electrodes capable of emitting secondary electrons, electrodemeans for producing an electrostatic field in radial direction, meansfor producing a magnetic field of substantially axial direction, acollecting output anode of rotational symmetry at one end of the coaxialelectrodes, and means for electrostatically controlling the currentflowing from the last of the electrodes emitting secondary electrons tothe output anode, such means comprising a grid of rotational symmetrybefore the output anode.

10. An electron multiplier comprising a plurality of substantiallycylindrical hollow elements having secondary-electron emissive innersurfaces, said cylindrical elements being mounted coaxially and axiallyspaced providing a column and the element at the input end of the columnbeing longer in the'axial direction than some of the remainingcylindrical elements, anode means positioned within said column andsubstantially coaxial with the cylindrical elements to provide asubstantially radial electric field, and means surrounding said anodefor producmg a substantially axial magnetic field, said magnetic fieldvarying in strength along the axis of the column within said longerelement.

11. An electron multiplier for the amplification of thermionic currentscomprising a plurality of secondary-electron emissive electrodes ofrotational symmetry, said electrodes being coaxial and axially spacedforming a column, a thermionic cathode of rotational symmetry in theax1s of the column at one end of it for supplying electrons to thesecondar -electron emissive electrode at said one end, said thermioniccathode being surrounded by an electrostatic controlling electrode ofrotational symmetry, anode means of rotational symmetry inside saidcolumn and coaxial therewith for producing an electrostatic field ofradial direction, means for producing a magnetic field of substantiallyaxial direction, means for producing a desired degree of inhomogeneityin said magnetic field compris mg. an annular ring of ferromagneticmaterial surrounding at least a part of one of the secondary-electronemissive electrodes, and a collecting output anode at the end of saidcolumn opposite said one end.

12. An electron multiplier comprising a plurality of rotationallysymmetrical secondaryelectron emissive electrodes for successivelymultiplymg a primary beam of electrons, said electrodes being coaxialand axially spaced forming a column, a thermionic cathode of' rotationalshape in the axis of said column at one end thereof for supplying anelectron beam to the secondary-electron emissive electrode at said oneend, ,an electrostatic controlling electrode of ro-.

tational symmetry around said thermionic cathode for controlling theintensity of said beam, and a plurality of deflecting disks mountedsubstantially parallel to the direction of said beam for varying theaxial position of incidence of said beam on the said emissive cathode atsaid one end.

13. An electron multiplier comprising a plurality of rotationallysymmetrical electrodes having secondary-electron emissive inner surfacesfor successively multiplying a primary beam of electrons, saidelectrodes being coaxial and axially spaced forming a column, anodemeans positioned within said column and substantially coaxial with saidemissive electrodes to provide a radial electric field, means forproducing a substantially axial magnetic field along said column, athermionic cathode of rotational shape in the axis of said column at oneend thereof for supplying a beam of electrons to the secondary-electronemissive electrode at said one end substantially throughout a rotationalarea about the axis of the column, an electrostatic controllingelectrode of rotational symmetry around said thermionic cathode forcontrolling the intensity of said beam, and a plurality of deflectingdisks mounted substantially parallel to the direction of said beam forvarying the axial position of incidence of said beam on the saidemissive cathode at said one end.

DENNIS GABoR.

